I'm not sure what the reasoning is at this point since i'm basing it solely on a headline... hopefully it wasn't the bullying although she wasn't exactly the dKos dream candidate either way.
OK so now there is some additional information.
“If nominated, I am now convinced that the confirmation process would be lengthy, disruptive and costly – to you and to our most pressing national and international priorities,” Rice wrote in a letter to President Obama, saying she’s saddened by the partisan politics surrounding her prospects.This is certainly accurate, and I honestly have no idea based on her comments if she was "forced out" or not. There's no reason to believe anyone should have actually been nominated by now (what with us currently having one) and if Benghazi is really the Republicans main objection I think that fight was winnable in the court of public opinion.
Whether through the administration's will or not, her withdrawal will probably gas up the Benghazi fire for a bit while conservatives insist some inside knowledge that will bring down the administration is her reason for dropping out.
Here's some additional richness for you:
Sen. Bob Corker (R-Tenn.) suggested Rice was seen as too much of an Obama loyalist and the GOP preferred “someone of independence.”It's hard for me to see anyone associated with Democratic politcs whom Bob Corker wouldn't refer to as an "Obama Loyalist." I hear Joe Lieberman has some free time.